sb/config: Add support for line continuation.

- Add line continuation support so empty macros can be used at
  the end of configure command lines, ie RTEMS kernel. The
  config parser removes empty lines and wihout this support a
  trailing line continuation character joins a configure command
  line to next command.

- Fix freetype's gmake support for non-GNU operating systems.

source-builder/config/freetype-1.cfg

@@ -41,8 +41,12 @@ URL:     http://www.freetype.org/
   %{host_build_flags}
 
   %ifos freebsd
-  GNUMAKE=%{__make} \
+   GNUMAKE=%{__make}
+  %else
+    GNUMAKE=
   %endif
+
+  GNUMAKE=${GNUMAKE} \
   ../${source_dir_freetype}/configure \
     --host=%{_host} \
     --prefix=%{_prefix} \

source-builder/sb/config.py

@@ -1056,20 +1056,32 @@ class file:
             line = line[0:-1]
             b = line.find('#')
             if b >= 0:
-                line = line[1:b]
+                line = line[1:b] + ('\\' if line[-1] == '\\' else '')
             return line.strip()
 
+        def _clean_and_pack(line, last_line):
+            leading_ws = ' ' if len(line) > 0 and line[0].isspace() else ''
+            line = _clean(line)
+            if len(last_line) > 0:
+                line = last_line + leading_ws + line
+            return line
+
         #
         # Need to add code to count matching '{' and '}' and if they
         # do not match get the next line and add to the string until
         # they match. This closes an opening '{' that is on another
         # line.
         #
+        ll = ''
         for l in config:
             self.lc += 1
-            l = _clean(l)
+            l = _clean_and_pack(l, ll)
             if len(l) == 0:
                 continue
+            if l[-1] == '\\':
+                ll = l[0:-1]
+                continue
+            ll = ''
             if isvalid:
                 indicator = '>'
             else: